1. Field of the Invention
The present invention relates to a traveling wave switch, and in particular to a traveling wave switch having FET-integrated CPW (coplanar waveguide) line structure.
2. The Prior Arts
In general, in wireless communication, the transmission/receiving switches play an important role in changing the channels of the signals of radio frequency (RF) from a transmitter to a receiver and vise versa. Recently, the switches utilizing the FET devices have become very popular and are widely used, as they can be realized by the standard manufacturing process and they can be integrated easily with other active components, such as the integrated power amplifiers or the low noise amplifiers. In order to raise the operation frequency of the switches thus they can be operated at high frequency, the design of a traveling wave shunt FET switch is proposed. In this design approach, the parasitic capacitance of the transistor and the parasitic inductance of the transmission line can be modeled as the low pass transmission line having specific impedance. Due to their broadband frequency response, thus switches based on traveling-wave concept are designed.
Usually, the operation frequency bandwidth of the traveling wave switch designed based on the traveling wave concept can be increased. However, when the signal frequency is greater than that of the W-band (75-110GHz), the parasitic inductance between the transistor in the switch and the signal line will restrict the operation frequency of the switch and its performance. In order to overcome this problem, a special manufacturing process of Hetero-junction FET (HJFET) is proposed, so that the operation frequency of the switch can be raised to 110GHz.
Regarding the standard manufacturing process of this type of traveling wave switch, a FET-integrated transmission line is proposed, which is used to eliminate the parasitic inductance between the transistor and signal line of this special structure. However, in this particular layout, the parasitic inductance between the device and ground still exists due to the existence of the via holes, and that will restrict the operation frequency of the traveling wave switch, and the details of which will be described in conjunction with an example as follows.
Firstly, please refer to FIG. 1 for a circuit diagram of an ordinary traveling wave switch of the prior art. As shown in FIG. 1, a resistor 13 is provided to control the voltage applied to the gate of a transistor 12, thus achieving the switching of the signal transmitted in the signal line 11 by turning on or turning off the transistor 12.
Next, referring to FIGS. 2(a) and 2(b). FIG. 2(a) is a schematic diagram of the structure of the traveling wave switch of the prior art. FIG. 2(b) is a circuit diagram of an equivalent circuit of the traveling wave switch shown in FIG. 2(a). As shown in FIG. 2(b), a parasitic inductance lp is created by a connection wire between the signal line 11 and the transistor 12; also, a parasitic inductance is created between the transistor 12 and ground due to the existence of a via hole 14 there-between, thus imposing restrictions on the switch so that its operation frequency can not be increased.
Then, referring to FIGS. 3(a) and 3(b). FIG. 3(a) is a schematic diagram of the structure of a traveling wave switch having FET-integrated transmission line of the prior art, which is an improvement of the traveling wave switch as shown in FIG. 2(a). FIG. 3(b) is a circuit diagram of the equivalent circuit of the traveling wave switch shown in FIG. 3(a). As shown in FIG. 3(b), the signal line 11 is connected directly to the source S of transistor 12, and the drain of the transistor is connected to ground. As such, in this configuration, the parasitic inductance lp created by the connection wire between the signal line 11 and the transistor 12 can be neglected. However, the parasitic inductance between the transistor 12 and ground still exists, that imposes a restriction on the traveling switch so that its operation frequency can not be increased.